1. Field of the Invention
The present invention relates to materials, methods, and systems for calibrating concentration measurements, and more particularly to materials, methods, and systems for calibrating surface impurity concentration measurements for semiconductor fabrication.
2. Description of the Related Art
Modern semiconductor fabrication techniques, including those for Very Large Scale Integration (VLSI), require extraordinary levels of cleanliness. In fact, reduction of trace contaminant levels is a primary goal of semiconductor fabrication and research facilities. Performance and yield are both adversely affected by trace impurity levels. Today, ultra-clean fabrication and research facilities demand surface contamination levels below 10.sup.10 atoms/cm.sup.2 and future facilities will require levels below 10.sup.9 atoms/cm.sup.2.
Consequently, the semiconductor industry, including researchers, wafer suppliers and device manufacturers, requires improved analytical techniques to measure organic and inorganic surface contaminants. Trace amounts of metallic contaminants are of particular concern and are known to seriously affect both performance and manufacturing yield of integrated circuits. To achieve the desired levels of cleanliness, extremely sensitive quantitative analysis techniques are required to measure impurity concentration levels. In turn, the equipment used in these techniques requires precise calibration. Examples of such quantitative analysis techniques include:
1. Total Reflection X-ray Fluorescence (TXRF), in which an X-ray beam strikes the surface to be examined at a grazing angle (below the "critical angle" or angle for total reflection) thus resulting in surface sensitivity of a few monolayers;
2. Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS), in which surface impurity atoms are sputtered off by a pulsed ion beam and subsequently detected by a time of flight mass spectrometer; and
3. Heavy Ion Backscattering (HIBS), in which a high mass, moderate energy ion beam is backscattered by surface atoms, the characteristics of the backscattered ions depending on the species of the surface scatterer.
Calibration of each of these quantitative analysis techniques for low-level impurity concentrations has proved difficult.
In general, calibration of equipment used in quantitative analysis techniques is necessary because of the variety of factors that affect the measurement, such as background noise, variations in detector and collection efficiency, uncontrolled sources of contamination, and variations in response from the impurity. Unfortunately, precise calibration of surface contamination measurement equipment typically requires the preparation of a tightly controlled reference standard with a precisely known impurity level. In this way, the actual response of the equipment can be calibrated against the known impurity level of a reference standard. Because it is difficult to precisely control the number of impurity atoms deposited on a reference standard, powerful analytical techniques have been used to characterize the reference standard before the reference standard can, itself, be used to calibrate other measurement equipment. For example, standards used for calibrating TXRF equipment have first been characterized by other methods, such as Atomic Absorption Spectrometry (AAS) or Inductively Coupled Plasma Mass Spectrometry ICP-MS. Unfortunately, many of these techniques suffer from their own calibration problems and from a similar lack of reference standards at concentration levels at or below 10.sup.10 atoms/cm.sup.2. As a result, precise calibration standards for surface concentration measurements in the range now desired (i.e., at or below 10.sup.10 atoms/cm.sup.2) are not generally available.
The problems inherent in many current calibration techniques can be illustrated in the context of TXRF, which is commonly used in the semiconductor industry to monitor the level of surface contamination on wafers at various stages of device fabrication. In order to provide a quantitative measurement of surface contamination levels, calibration curves have been measured based on well controlled standard samples with known impurity concentration. However, current calibration techniques have proved unreliable at surface contamination levels on the order of 10.sup.10 atoms/cm.sup.2. Round-robin measurements have been used with only marginal success in an attempt to mask inconsistencies among characterizations of reference standards. For example, round-robin test results reported to the ISO/TC 201/WG2 on Total X-ray Fluorescence Spectroscopy indicated inconsistencies in calibration data measured by a variety of different researchers despite the fact that they were using the same method. See e.g., Third Working Draft, Surface chemical analysis--Determination of contamination elements contents on silicone wafer--Total reflections X-ray fluorescence spectroscopy (TXRF), ISO/TC 201/WG2 N 26, Nov. 28, 1995. Chief among the report's concerns were the accuracy of initial measurements of the impurity concentration and unintended contamination from external sources. These problems are compounded when contamination takes the form of the same impurity atom intentionally used in the standard, or when the originally known contamination levels decrease unpredictably. Annex C of the above report specifies compositions for, procedures for preparing, and methods for calibrating a TXRF measurement apparatus with a standard specimen.